I would suggest replacing a bearing any time you have it out. It may be undamaged but have a small amount of wear that cannot yet be detected. No bigger bummer than having to tear it apart to replace the bearing 6 months down the road.
I recently did my honda weel bearings and learned a expensive lesson. I have done my fair share on atv,cars etc but no bikes. I used heat and puch method to remove them and did not pay close enough attention. The bearing went slightly off center and cracked the hub. Well $450 dollars later it's fixed but it hurt...real bad.